In Situ Identification of Nanoparticle Structural Information Using Optical Microscopy.

Diffraction-limited optical microscopy lacks the resolution to directly characterize nanoscale features of single nanoparticles. This paper describes how structural features of gold nanostars can be identified using differential interference contrast (DIC) microscopy. First, we established structure-property relationships between categories of nanoparticle shapes and DIC optical images and then validated the correlation with electrodynamic simulations and electron microscopy. We found that DIC image patterns of single nanostars could be differentiated between 2D and 3D geometries. DIC images were also used to distinguish asymmetric and 4-fold symmetric structures and track nanoparticle orientation. Finally, we demonstrated how this wide-field optical technique can be used for in situ characterization of single nanoparticles rotating at a glass-water interface.

Wavelength-Dependent Differential Interference Contrast Inversion of Anisotropic Gold Nanoparticles.

Gold nanorods are promising nanoparticle-orientation sensors because they exhibit wavelength and angle-dependent optical patterns in their differential interference contrast (DIC) microscopy images. In this paper, we report a finite-difference time-domain method to simulate DIC images using nanorods as model probes. First, we created a DIC image library of nanorods as a function of imaging wavelength and rotation angle that showed good agreement with experimental results. Second, we used this simulation tool to explain why the patterns inverted from bright to dark when the imaging wavelength increased from below to above the plasmon resonance of the nanorod. We found that this intensity inversion resulted from reversal in electric field direction depending on wavelength relative to the nanorod plasmon resonance. Finally, we showed that this DIC contrast inversion is a general phenomenon by measuring and simulating DIC images from gold nanorods of different sizes and gold nanostars.

Sequential Feature-Density Doubling for Ultraviolet Plasmonics.

Patterning of nanostructures with sub-200 nm periodicities over cm2-scale areas is challenging using standard approaches. This paper demonstrates a scalable technique for feature-density doubling that can generate nanopatterned lines with periodicities down to 100 nm covering >3 cm2. We developed a process based on controlled wet overetching of atomic-layer deposited alumina to tune feature sizes of alumina masks down to several nm. These features transferred into silicon served as masters for template-stripping aluminum nanogratings with three different periodicities. The aluminum nanogratings supported surface plasmon polariton modes at ultraviolet wavelengths that, in agreement with calculations, depended on periodicity and incident excitation angle.

Programmable and reversible plasmon mode engineering.

Plasmonic nanostructures with enhanced localized optical fields as well as narrow linewidths have driven advances in numerous applications. However, the active engineering of ultranarrow resonances across the visible regime-and within a single system-has not yet been demonstrated. This paper describes how aluminum nanoparticle arrays embedded in an elastomeric slab may exhibit high-quality resonances with linewidths as narrow as 3 nm at wavelengths not accessible by conventional plasmonic materials. We exploited stretching to improve and tune simultaneously the optical response of as-fabricated nanoparticle arrays by shifting the diffraction mode relative to single-particle dipolar or quadrupolar resonances. This dynamic modulation of particle-particle spacing enabled either dipolar or quadrupolar lattice modes to be selectively accessed and individually optimized. Programmable plasmon modes offer a robust way to achieve real-time tunable materials for plasmon-enhanced molecular sensing and plasmonic nanolasers and opens new possibilities for integrating with flexible electronics.

Unidirectional Lasing from Template-Stripped Two-Dimensional Plasmonic Crystals.

Plasmon lasers support cavity structures with sizes below that of the diffraction limit. However, most plasmon-based lasers show bidirectional lasing emission or emission with limited far-field directionality and large radiative losses. Here, we report unidirectional lasing from ultrasmooth, template-stripped two-dimensional (2D) plasmonic crystals. Optically pumped 2D plasmonic crystals (Au or Ag) surrounded by dye molecules exhibited lasing in a single emission direction and their lasing wavelength could be tuned by modulating the dielectric environment. We found that 2D plasmonic crystals were an ideal architecture to screen how nanocavity unit-cell structure, metal material, and gain media affected the lasing response. We discovered that template-stripped strong plasmonic materials with cylindrical posts were an optimal cavity design for a unidirectional laser operating at room temperature.

Quasiperiodic moiré plasmonic crystals.

This paper describes the properties of silver plasmonic crystals with quasiperiodic rotational symmetries. Compared to periodic plasmonic crystals, quasiperiodic moiré structures exhibited an increased number of surface plasmon polariton modes, especially at high angles of excitation. In addition, plasmonic band gaps were often formed at the intersections of these new modes. To identify the origin and predict the location of the band gaps, we developed a Bragg-based indexing system using the reciprocal lattice vectors of the moiré plasmonic crystals. We showed that even more complicated quasiperiodic geometries could also be described by this indexing model. We anticipate that these quasiperiodic lattices will be useful for applications that require the concentration and manipulation of light over a broadband spectrum.

Polymer nanowrinkles with continuously tunable wavelengths.

This paper describes a parallel method to generate polymer nanowrinkles over large areas with wavelengths that were continuously tuned down to 30 nm. Reactive ion etching using fluorinated gases was used to chemically treat thermoplastic polystyrene films, which resulted in a stiff skin layer. Upon heating, the treated thermoplastic, microscale, and nanoscale wrinkles were formed. We used variable-angle spectroscopic ellipsometry to characterize the thickness of the skin layer; this thickness could then be used to predict and control the nanowrinkle wavelength. Because the properties of these nanotextured polymer surfaces can be tuned over a large range of wrinkle wavelengths, they are promising for a broad range of applications, especially those that require large-area and uniform surface patterning.

High-rotational symmetry lattices fabricated by moiré nanolithography.

This paper describes a new nanofabrication method, moiré nanolithography, that can fabricate subwavelength lattices with high-rotational symmetries. By exposing elastomeric photomasks sequentially at multiple offset angles, we created arrays with rotational symmetries as high as 36-fold, which is three times higher than quasiperiodic lattices (≤12-fold) and six times higher than two-dimensional periodic lattices (≤6-fold). Because these moiré nanopatterns can be generated over wafer-scale areas, they are promising for a range of photonic applications, especially those that require broadband, omnidirectional absorption of visible light.

Atomic layer deposition-based functionalization of materials for medical and environmental health applications.

Nanoporous alumina membranes exhibit high pore densities, well-controlled and uniform pore sizes, as well as straight pores. Owing to these unusual properties, nanoporous alumina membranes are currently being considered for use in implantable sensor membranes and water purification membranes. Atomic layer deposition is a thin-film growth process that may be used to modify the pore size in a nanoporous alumina membrane while retaining a narrow pore distribution. In addition, films deposited by means of atomic layer deposition may impart improved biological functionality to nanoporous alumina membranes. In this study, zinc oxide coatings and platinum coatings were deposited on nanoporous alumina membranes by means of atomic layer deposition. PEGylated nanoporous alumina membranes were prepared by self-assembly of 1-mercaptoundec-11-yl hexa(ethylene glycol) on platinum-coated nanoporous alumina membranes. The pores of the PEGylated nanoporous alumina membranes remained free of fouling after exposure to human platelet-rich plasma; protein adsorption, fibrin networks and platelet aggregation were not observed on the coated membrane surface. Zinc oxide-coated nanoporous alumina membranes demonstrated activity against two waterborne pathogens, Escherichia coli and Staphylococcus aureus. The results of this work indicate that nanoporous alumina membranes may be modified using atomic layer deposition for use in a variety of medical and environmental health applications.